NOAA photoBluefin tuna are international travelers. They leave their spawning grounds in the Gulf of Mexico early each spring and travel north along the U.S. Atlantic coast to the shores of Massachusetts and Nova Scotia, Canada.View full sizeThe bluefin tuna tracked

The 700-pound giant Atlantic bluefin tuna carrying satellite radio tag number 5108024 entered the Gulf of Mexico on March 23, 2009, hugging the coast of Cuba, and speeding along a straight line to the warm water body's center.

The huge fish meandered north toward what would become its spawning and feeding territory over the next two months, the deep Gulf slope between Louisiana and western Florida. Finally, it left the Gulf for colder water in the Atlantic on May 24. Its tag was jettisoned for pickup by scientists a few days later.

More than once, the fish's squiggly path took it right across the mouth of the Mississippi River, alongside the drilling ship developing BP's Deepwater Horizon well.

If the fish had been tagged this year, the tag would likely have shown the tuna following the same path, where it and its eggs would have been swimming or floating in the oil being released from the sunken Deepwater Horizon rig, said Barbara Block, a Stanford University marine biologist who has participated in the tagging of more than 1,000 bluefins during the past 10 years.

"It's hot in the Gulf, and these are big, warm-bodied fish," she said. "They go from the frigid waters off Canada to the Gulf, and they love to go right to where that oil rig was."

Block said the public seems fixated on the effects of the oil release on wetlands and the shoreline.

"But the tragedy that's really unfolding in the Gulf is that this is springtime, a time of renewal for many of the ocean species, and this is an important nursery ground for the larval forms of many species, bluefin and yellowfin tuna, swordfish, and a variety of others," she said.

"The most delicate stage of life is the larval stage of any fish, and any fish floating in oil is probably not a happy larva," she said. "I would urge BP and others involved in this oil incident to invest in finding out the effect of this oil on the larval fish of North America."

View full sizePhoto courtesy of Stanford Tuna Research and Conservation CenterSteve Wilson of Stanford University installs a satellite radio tag on a 700-pound Atlantic bluefin tuna in October 2008. The fish entered the Gulf of Mexico on March 23, 2009, to spawn, and returned to the Atlantic on May 24, 2009, a journey that took it near the Deepwater Horizon rig site.
During their two months in the Gulf, the huge tuna reach their warmest body temperatures of the year, as they expend energy to court, mate and spawn, each female laying as many as a million eggs. The resulting stress means the tuna will breathe much more rapidly than usual, and their gills are much more likely to capture tiny droplets of oil suspended in the water column, the result of dispersants used to break up the oil spill, Block said.

Even under the best of conditions, only a few eggs from each spawning fish survive, with most being taken by predators or storms or other natural events. But the dispersants being used on the oil also are likely to break up the surface tension of the water, possibly causing the eggs to sink to much deeper levels, where they're less apt to survive. If oiled or covered with dispersant, the eggs themselves may be destroyed.

"They're skimming the surface in that mixed layer right where these animals are putting their eggs and larvae," she said. "It's an insult this fish can't afford at this time, when they're really considered an endangered species in the same vein as pandas, tigers and rhinoceroses."

The bluefin is at the top of the Gulf's food chain, and that means its lifespan is much longer than shrimp, which live for eight or nine months, or the other smaller creatures on which it feeds. Just growing to sexual maturity takes as long as 10 years, which means the loss of a significant number of fish from a single year-class can threaten the species, she said.

"When you lose a year class, the effects come to bear on the fishery 10 years from now, when you don't have enough fish to spawn a new generation," she said.

And while commercial fishers are prohibited from targeting bluefins in the Gulf of Mexico, they're still allowed to use longline gear to target other species, such as yellowfin tuna and swordfish, and often capture bluefins as bycatch on those lines.

Indeed, the Atlantic bluefin should be on a United Nations list of species so overfished that their capture everywhere for sale in the international marketplace would be banned, Block said. A proposal to do just that in March, supported by the U.S. government, was blocked by Japan and several African nations that benefit from the sale of the coveted fish. In January, a 513-pound Pacific bluefin tuna sold for a record $177,000 at the Tsukiji Fish Market in Tokyo.

Block is not the only scientist concerned about the effects of oil on fish in deeper waters.

"This couldn't happen at a worse time for bluefin," said Lee Crockett, director of the Pew Environmental Group's Federal Fisheries Policy Reform Project. "If this leak goes on for months, as is possible, you're going to have blue and white marlin coming in to spawn in the same area."

Roger Zimmerman, director of the Galveston Marine Laboratory of the National Marine Fisheries Service, a division of NOAA, said he hopes information being gathered to determine the natural resource damage caused by the well blowout will broaden the understanding of deep-ocean habitats.

"If we were talking about oil going into marshes and wetlands, we have lots of information on how petroleum affects the intertidal zone and marshes and wetlands," Zimmerman said. "But we don't have much information on the magnitude of effects that transpire when oil is impacting habitat in deep waters on coral reefs or on the continental shelf. But obviously, they're going to be negative impacts, impacts that are not necessarily very good."